Voltage sensors for high voltage power line systems may be realized by the use of one or more electric field sensors. Further, such electric field sensors may be optical electric field sensors. The advantages of employment of optical sensors in power line applications are well known. Specifically a very significant advantage is that when optical fibers are employed as the signal-carrying medium, information contained in an optical wave is generally not affected by the severe electrical environment found in the power line systems environment—i.e., the power station.
Examples of optical electric field sensors are shown and described in U.S. Pat. No. 6,252,388, entitled, “Method and Apparatus for Measuring Voltage Using Electric Field Sensors,” and U.S. Pat. No. 6,380,725, entitled, “Voltage Sensor,” both of which are incorporated herein by reference. As illustrated therein, one or more electric field sensors are placed within an insulator column extending between ground potential and the voltage to be measured. The positions of the electric field sensors may be chosen according to a Gaussian-quadrature formulation, and the electric field sensors may be placed within a resistive shield that acts to grade the electric field, smoothing out high spatial electric field variations which may be generated on the surface of the insulator in polluted and/or wet environments.
Further, an example of an optical electric field sensor employed as a voltage sensor is shown and described U.S. Pat. No. 4,939,447, entitled, “Fiber Optic Voltage Sensor”, issued to Bohnert, et al, in which a voltage sensor uses multiple fiber-optic piezoelectric electric field sensors to measure voltage. Accuracy is obtained by the use of a plurality of sensors; to obtain reasonable accuracy 22 sensors connected in series were required.
Another example of an optical electric field sensor is a Pockels cell for measuring voltage, particularly high voltage, as taught in U.S. Pat. No. 5,477,134, entitled, “Voltage Sensor For Use In Optical Power Transformer Including A Pair Of Pockels Cells, issued to Hamada,” and U.S. Pat. No. 5,731,579, entitled, “Electro-Optical Voltage Sensor Head,” issued to Woods, both of which are herein incorporated by reference thereto. Pockels cells may be constructed in the form of an integrated optics Pockels cell such as that described in U.S. Pat. No. 5,029,273, entitled, “Integrated Optics Pockels Cell Voltage Sensor,” issued to Jaeger, which is also incorporated herein by reference thereto.
As is well understood in present day three-phase power stations, commonly there are three insulator columns, each of which includes a voltage transformer for deriving an indication of voltage of the high voltage on the power line. Analogously, electric field sensors may replace the voltage transformers for deriving a measurement of the power line voltage. If multiple electric field sensors are employed in each insulator column to derive the power line voltage associated with a single power line, each of the sensors must be interrogated to determine the strength of the electric field across the corresponding electric field sensor. In the scenario where optical electric field sensors are employed, the interrogation is accomplished by observing the behavior of optical waves in the presence of an electric field at the specific sensor location. In the following exposition, it should be assumed that the electric field sensors are optical electric field sensors.
Generally, associated with each optical electric field sensor is a remote dedicated electronic/optical sensor circuit module that receives an optical signal from a respective one of the optical electric field sensors that may form, in part, an array of electric field sensors intended to measure voltage associated with one phase of a three-phase power line system. The circuit module generally may include (i) a light source for delivering a light wave to the sensor, and a pair of combination optical signal detectors and signal converters for converting a pair of optical signals associated with a sensor into a pair of electrical signals for subsequent signal processing. It should be understood that the aforesaid sensor circuit module may be implemented on one circuit card, or may alternatively be on a circuit card embodying multiple sensor circuit modules or other electronic circuits or circuit modules.
Before proceeding, it should be recognized that preferably, to realize a voltage sensor, the aforesaid pair of electrical signal outputs from the converter may be combined by way of a signal processor for estimating the total voltage across the insulator using a selected optical signal algorithm, i.e., sums the results of multiple optical electric field sensors (e.g., three as will be assumed in the following exposition) in accordance with an algorithm, e.g., selected weighting of the multiple electric field sensors as taught in the aforementioned U.S. Pat. No. 6,252,388.
Of course, numerous control and characterization algorithms may be utilized in the processing of the raw optical signals, and corresponding detector output electrical signals, in the computation of the final voltage measurement. These algorithms may accomplish temperature characterization and also correct for a host of other optical/electrical/mechanical component parameter changes.
The total amount of electronics and fiber optic links required to realize a power line voltage sensor employing multiple optical electric field sensors can be a basic shortcoming of the prior art depending on the particular implementation. For example, as taught in the aforementioned power line voltage sensors, one input fiber and two output fibers will generally be required for each of three optical electric field sensors, leading to nine (9) optical fiber links per voltage sensor, or 27 fiber links for a three phase system. Such a system may then require three sensor circuit modules, one associated with each of the three electric field sensors, in the example, plus one additional circuit module for signal processing that performs, in part, the signal combining functions.
It should be recognized, however, that one way to reduce the number of optical fibers in the just described system, as well as the number of circuit modules, is to cascade the sensors—passing a light wave successively through multiple sensors. This arrangement has been mentioned in the aforerecited U.S. Pat. Nos. 6,252,388 and 6,380,725, however, such an arrangement is difficult to implement and has various limitation and restrictions.
It should be further noted that (i) one electronics board or circuit card (or equivalent electronic hardware) is generally required for each of the multiple optical electric field sensors (e.g., three)—each possibly containing a light source, and (ii) a dedicated optical signal combiner circuit card or module (or equivalent circuit) for summing the results of the optical electric field sensors. Thus, four dedicated electronic circuits or electronic cards would be required for each voltage sensor employing three optical electric field sensors, or 12 dedicated electronics circuits or electronic cards would be required for a three-phase system.
Accordingly, there is a need for a method and apparatus for minimizing the number of dedicated electronic circuits and/or circuit cards, and reducing the number of optical fiber links and number of electronics boards, while keeping the basic architecture of using multiple appropriately weighted electric field sensors to estimate power line voltage—i.e., across the high voltage insulator column from the power line to ground, particularly for one phase of a three-phase power line system.